The USGS drill rig on Cockspur Island, February 2010. Colleen T. Durand, USGS.
The island and the fort
Fort Pulaski was built from 1829 to 1847 and made a national monument by President Calvin Coolidge in 1924. It stands on Cockspur Island in Chatham County, Georgia, in the Atlantic Coastal Plain, near the mouth of the Savannah River. The island is made of small hummocks, salt marshes and sediment dredged from the river.
(A) Fort Pulaski and the Floridan aquifer system (green), modified from Williams and Kuniansky (2016). (B) Fort Pulaski National Monument; the star marks the drill site. North of Cockspur Island, the middle of the Savannah River is the Georgia–South Carolina line. USGS.
What lies beneath
An earlier study described a 116.5-foot (35.5 m) core from the island's north side. In 2010, the U.S. Geological Survey drilled a 1,017-foot (310 m) core west of the fort. From top to bottom it passes through five units:
| Unit | Thickness | Age |
|---|---|---|
| Sand and mud | 53 ft (16 m) | present to 4,300 years |
| Sand and mud with phosphate | 35 ft (11 m) | 3.5 to 12 million years |
| Sand and mud | 94 ft (29 m) | 12 to 32 million years |
| Limestone (including ~25 ft [7.6 m] of sandy limestone) | 783 ft (239 m) | 32 to 41 million years |
| Glauconitic sand | 53 ft (16 m) | 41 to 43 million years |
The limestone and the water in it make up the Floridan aquifer system — the main source of drinking water for Savannah and nearby communities.
The Cockspur Island core, by depth, with its formations and approximate ages — from the Satilla Formation at the top to the Marley Mill Formation at the bottom. Ages from USGS biozone studies and radiocarbon dating. USGS.
Part of the core from 385.5 to 393.4 feet (117.5 to 119.9 m) deep. Wilma Alemán-González, USGS.
What the fort is made of
Fort Pulaski's builders brought materials from near and far.
Granite. Mercer Parker, USGS.
- Granite: stairs, tops of brick posts, walls and the lining of the old wharf at the Historic North Pier. An igneous rock formed as molten magma cooled, most of it was quarried in New England, shipped via New York, and brought to the fort by sea. The fort has four varieties, perhaps 280 million to 1 billion years old.
Bricks. Mercer Parker, USGS.
- Bricks, of two kinds:
- "rose-red", for arches and interior walls, made from mud and sand dug near the fort;
- "rose-brown", for the exterior walls and the walls lining the moat, made in Baltimore and Alexandria, probably from heavily weathered metamorphic rock.
Sandstone. Mercer Parker, USGS.
- Sandstone: brownish-red blocks bordering the moat and lining doorway thresholds and brick arches. A sedimentary rock of sand-size grains, it was quarried in Connecticut from the Portland Formation of the Newark Supergroup — "Portland brownstone" — and is about 190 to 200 million years old, laid down in basins on land as the Atlantic opened and the supercontinent Pangaea split apart.
- Lime mud with oyster shells — "tabby": the cement of the walkway through the demilune moat, used as mortar and as a coating over bricks. Shells were crushed and burned to make lime, mixed with water, sand, wood ash and more crushed shell, poured into wooden molds and left to harden — a method used in many coastal forts in the 1800s. The shells probably came from Native American middens near the Savannah River.
Sources
Based on A Step Back in Time—A Brief Geological History of Cockspur Island at Fort Pulaski National Monument, Chatham County, Georgia, USGS Fact Sheet 2018–3011, U.S. Geological Survey; a work of the United States government in the public domain. The maps, core description and USGS photographs are taken from the fact sheet's PDF; its other photographs, several under Creative Commons licenses, are not reproduced. The imported text had the sentences on bricks out of order; this page follows the fact sheet.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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